歪制御低サイクル疲労における平均歪の影響

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Taking the mean strain and strain ratio as parameters, diametal strain controlled low cylce fatigue tests were carried out on hour-glass shaped specimens of two quenched and tempered 60 kg/mm^2 high tensile steels.As a result of the tests, the following conclusions were obtained;1. The effect of the mean strain to the crack initiation life was not significant as far as the mean strain was not more than 10% of the static ductility. For the crack initiation life less than a few cycle fatigue strenght, but for longer life the Gerber's law is good to express the relation between mean strain and low cycle fatigue strenght.2. On the basis of the hysteresis energy consideration, an equation to estimate the effect of meanstrain to crack initiation life was proposed as follows; 1-N_c/N_<c0>=[W_<1/4>(ε_<tm>+ε_<pa>)-W_<1/4>(ε_<pa>)]/W_f where N_c is a number of cycles to crack initiation when mean strain is ε_<tm>, N_<c0> is a number of cycles to crack initiation when mean strain is zero, W_<1/4>(ε_<tm>+ε_<pa>) and W_<1/4>(ε_<pa>) are energy required to strain to ε_<tm>+ε_<pa> and ε_<pa>, respectively, and W_f is energy required to static fractur. The equation gives better estimation for crack initiation life more than 500 cycles than the equations proposed by Weiss et al., Munse et al., and Ohji et al.3. A simple method to estimate the mean strain at the first cycle was proposed. The mean stress at the first cycle is rapidly relaxed by strain cycling, and the behavior of the mean stress is well approximated by the foloowing equation;σ^<-(m-1)>-σ_0^<-(m-1)>=(m-1)BEn. where σ is mean stress at the n th cycle, σ_0 is a constant determined by initial condition, m is a constant which depends on cycling rate and temperature, BE is a function of strain amplitude, cycling rate and temperature, and n is a number of cycles imposed.4. Hysteriesis energy per cycle is influenced by the mean strain at the beginning of strain cycling, but not after the mean stress is relaxed.5.Crack growth life is not influenced by the mean strain because the mean stress is relaxed before a crack initiates.6.Crack initiation life is increased by the heat treatment of stress selieving by 16%, but crack growth life is not influenced.
社団法人日本船舶海洋工学会の論文

歪制御低サイクル疲労における平均歪の影響

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